This invention relates to scanning probe microscopes, which are used to obtain extremely detailed analyses of the topography and other characteristics of samples such as semiconductor devices and data storage media and, more particularly, to scanning probe microscopes which are classified as scanning force microscopes or scanning tunneling microscopes.
xe2x80x9cScanning probe microscopexe2x80x9d (SPM) means an instrument which provides a microscopic analysis of the topographical features or other characteristics of a surface by causing a probe to scan the surface. It refers to a class of instruments which employ a technique of mapping the spatial distribution of a surface property, by localizing the influence of the property to a small probe. The probe moves relative to the sample and measures the change in the property or follows constant contours of the property. Depending on the type of SPM, the probe either contacts or rides slightly (up to a few hundred Angstroms) above the surface to be analyzed. Scanning probe microscopes include devices such as scanning force microscopes (STMs), scanning tunneling microscopes (STMS), scanning acoustic microscopes, scanning capacitance microscopes, magnetic force microscopes, scanning thermal microscopes, scanning optical microscopes, and scanning ion-conductive microscopes.
xe2x80x9cProbexe2x80x9d means the element of an SPM which rides on or over the surface of the sample and acts as the sensing point for surface interactions. In an SFM the probe includes a flexible cantilever and a microscopic tip which projects from an end of the cantilever. In an STM the probe includes a sharp metallic tip which is capable of sustaining a tunneling current with the surface of the sample. This current can be measured and maintained by means of sensitive actuators and amplifying electronics. In a combined SFM/STM the probe includes a cantilever and tip which are conductive, and the cantilever deflection and the tunneling current are measured simultaneously.
xe2x80x9cCantileverxe2x80x9d means the portion of the probe of an SFM which deflects slightly in response to forces acting on the tip, allowing a deflection sensor to generate an error signal as the probe scans the surface of the sample.
xe2x80x9cTipxe2x80x9d in an SFM means the microscopic projection from one end of the cantilever which rides an or slightly above the surface of the sample. In an STM, xe2x80x9ctipxe2x80x9d refers to the metallic tip.
xe2x80x9cPackagexe2x80x9d means an assembly which includes the cantilever and tip, a chip from which the cantilever projects, and may include a plate on which the chip is mounted.
xe2x80x9cScanning Force Microscopexe2x80x9d SFM (sometimes referred to as Atomic Force Microscope) means an SPM which senses the topography of a surface by detecting the deflection of a cantilever as the sample is scanned. An SFM may operate in a contacting mode, in which the tip of the probe is in contact with the sample surface, or a non-contacting made, in which the tip is maintained at a spacing of about 50 xc3x85 or greater above the sample surface. The cantilever deflects in response to electrostatic, magnetic, van der Waals or other forces between the tip and surface. In these cases, the deflection of the cantilever from which the tip projects is measured.
xe2x80x9cScanning Tunneling Microscopexe2x80x9d (STM) means an SPM in which a tunneling current flows between the probe and the sample surface, from which it is separated by approximately 1-10 xc3x85. The magnitude of the tunneling current is highly sensitive to changes in the spacing between the probe and sample. STMs are normally operated in a constant current mode, wherein changes in the tunneling current are detected as an error signal. A feedback loop uses this signal to send a correction signal to a transducer element to adjust the spacing between the probe and sample and thereby maintain a constant tunneling current. An STM may also be operated in a constant height mode, wherein the probe is maintained at a constant height so that the probe-sample gap is not controlled, and variations in the tunneling current are detected.
xe2x80x9cKinematic mountingxe2x80x9d means a technique of removably mounting a rigid object relative to another rigid object so as to yield a very accurate, reproducible positioning of the objects with respect to each other. The position of the first object is defined by six points of contact an the second. These six points must not over or under constrain the position of the first object. In one common form of kinematic mounting, three balls on the first object contact a conical depression, a slot (or groove) and a flat contact zone, respectively, on the second object. Alternatively, the three balls fit snugly within three slots formed at 120xc2x0 angles to one another on the second object. The foregoing are only examples; numerous other kinematic mounting arrangements are possible. According to the principles of kinematic mounting, which are well known in the mechanical arts, six points of contact between the two objects are required to establish a kinematic mounting arrangement. For example, in the first illustration given above, the first ball makes contact at three points on the conical surface (because of inherent surface imperfections, a continuous contact around the cone will not occur), two points in the slot, and one point on the flat surface, giving it a total of six contact points. In the second illustration, each ball contacts points on either side of the slot into which it fits.
Scanning probe microscopes (SPMs) are used to obtain extremely detailed analyses of the topographical or other features of a surface, with sensitivities extending down to the scale of individual atoms and molecules. Several components are common to practically all scanning probe microscopes. The essential component of the microscope is a tiny probe positioned in very close proximity to a sample surface and providing a measurement of its topography or some other physical parameter, with a resolution that is determined primarily by the shape of the tip and its proximity to the surface. In a scanning force microscope (SFM), the probe includes a tip which projects from the end of a cantilever. Typically, the tip is very sharp to achieve maximum lateral resolution by confining the force interaction to the end of the tip. A deflection sensor detects the deflection of the cantilever and generates a deflection signal, which is then compared with a desired or reference deflection signal. The reference signal is then subtracted from the deflection signal to obtain an error signal, which is delivered to a controller. There are several types of deflection sensors. One type uses an optical interferometer as described in an article by D. Rugar et al., Review of Scientific Instruments, Vol. 59, p. 2337 (1988). Most commercial SFMs, however, employ a laser beam which is reflected from the back of the cantilever and use a photodetector to sense the angular movement of the beam as the cantilever is deflected. The probe (cantilever and tip) and deflection sensor are normally housed in a unit referred to as a head, which also contains circuitry for preamplifying the signals generated by the deflection sensor before they are passed to a controller. An image is formed by scanning the sample with respect to the probe in a raster pattern, recording data at successive points in the scan, and displaying the data on a video display. The development of scanning (or atomic) force microscopy is described in articles by G. Binnig et al., Europhys. Lett., Vol. 3, p. 1281 (1987), and T. R. Albrecht et al., J. Vac. Sci. Technology, A6, p. 271 (1988). The development of the cantilever for SFMs is described in an article by T. R. Albrecht et al., entitled xe2x80x9cMicrofabricated Cantilever stylus for Atomic Force Microscopyxe2x80x9d. J.Vac. Sci. Technol., A8, p. 3386 (1990). Other types of SPMs, such as scanning capacitance or scanning magnetic force microscopes, also use similar deflection sensors.
A scanning tunneling microscope (STM) is similar to an SFM in overall structure, but the probe consists of a sharpened conductive needle-like tip rather than a cantilever. The surface to be mapped must generally be conductive or semiconductive. The metallic needle is typically positioned a few Angstroms above the surface. When a bias voltage is applied between the tip and the sample, a tunneling current flows between the tip and the surface. The tunneling current is exponentially sensitive to the spacing between the tip and the surface and thus provides a representation of the spacing. The variations in the tunneling current in an STM are therefore analogous to the deflection of the cantilever in an SFM. The head contains circuitry for biasing the tip with respect to the sample and preamplifying the tunneling current before it is passed to a controller.
Before a desired region on the sample can be analyzed in an SFM, it must be positioned properly with respect to the probe, that is, the probe must be positioned above the location on the sample to be examined and must be brought into contact or close proximity with the sample. This requires two types of movement: first a lateral (x, y) movement and then a vertical (z) movement. The translations required to do this are beyond the limited range of the x, y, z fine movement stage. This process may be accomplished manually or with xe2x80x9ccoarsexe2x80x9d positioning stages. In the latter case, the sample or head is mounted on a coarse x, y stage, which is capable of horizontal movement in any direction to properly position the sample beneath the probe. Typically, a coarse x, y stage has a translation range of around 25 mm.
A coarse z stage is used to position the probe vertically with respect to the sample. It is desirable that a z coarse stage permit maximum sample-probe separation (e.g., 30 mm or more if possible). In this position, the probe can be changed if necessary and/or a different sample may be placed in the SPM. The coarse z stage is also adjustable to bring the probe to a distance (e.g., of around 100 xcexcm) where the position relationship between the probe and sample can be viewed through an accessory optical microscope. The coarse x, y stage is then used to move the sample horizontally with respect to the probe until the optical microscopic view indicates that the probe is positioned over a feature or area of the sample which is to be analyzed. The coarse z stage is then adjusted carefully so as to bring the probe to the sample gradually until the scanner fine x, y, z stage (scanner, described below) and its associated feedback loop (described below) take over to maintain a proper probe-sample separation. The final approach requires a resolution of about one micron and must be performed delicately to avoid crashing the probe into the sample.
In all of the coarse and fine (scanning) movements, the key factor is the position and movement of the probe relative to the sample. The actual movement may be performed by the probe or the sample or both.
The scanning operation is performed by a fine x, y, z stage, or scanner, which has a range of about 1-300 xcexcm in the x and y directions and about 1-15 xcexcm in the z direction. The scanner typically moves the sample horizontally such that the probe follows a raster-type path over the surface to be analyzed. In the fast scan direction, a computer collects a line of data at a series of points. Movement in the slow scan direction positions the scanner for the next line of data points to be taken. The resulting image will be made up of individual pixels. Usually, all data are collected in the same fast scan direction, that is, data are not collected along the reverse path.
In most SPMs, the scanning movement is generated with a vertically-oriented piezoelectric tube. The base of the tube is fixed, while the other end, which may be connected to either the probe or the sample, is free to move laterally as an input voltage signal is applied to the piezoelectric tube. The use of a piezoelectric tube in this application is well known and is described, for example, in an article by Binnig and Smith, Review of Scientific Instruments, Vol. 57, pp. 1688 (August 1986).
Fine movement in the z direction is normally also obtained using a piezoelectric device. FIG. 11A illustrates the prior art feedback loop for controlling the movement of the scanner in the z direction. Assuming the device is an SFM or other device that uses a similar type of cantilever, a deflection sensor measures the deflection of the probe and generates an error signal E which is the difference between the deflection signal and a reference signal. The error signal E is passed to a controller which applies a z feedback voltage signal Zv which drives a scanner in the z direction so as to maintain a constant cantilever deflection as the sample is scanned horizontally. For example, if the probe encounters a bump in the sample surface, the feedback signal Zv will cause the scanner to increase the separation between the probe and the sample and thereby maintain a constant cantilever deflection. The feedback signal Zy thus represents the sample topography and can be used to form an image. Alternatively, the SFM nay be operated with the x feedback adjusted so as to compensate only for large topographical features such as sample slope, and the error signal E may be used to generate a representation of the sample surface. This mode has disadvantages. For example, damage to the surface or probe may occur f the probe deflection exceeds a maximum limit.
In the prior art the function of the controller may be achieved purely by analog circuitry, in which the error signal is appropriately processed in order to optimize the performance of the feedback loop. Alternatively, the error signal may be digitized, and the processing may be performed digitally using a computer or digital signal processing device, such as are commonly known and available. In the latter case, the digital signals are converted back into analog form before they are transmitted to the scanner.
The feedback loop in an STM operates in a very similar manner, the primary difference being that the error signal which is sent to the controller is generated by the tunneling current rather than the deflection of a cantilever. The difference of this current from a set value, which is a function of the spacing between the probe and the surface, is used by the controller to determine the z feedback signal which it sends to the scanner. The feedback signal adjusts the scanner position to maintain constant spacing between the probe and the surface. Since the tunneling current depends exponentially on the spacing between the probe and the surface, a high vertical sensitivity is obtained. Because the probe may be atomically sharp, the lateral sensitivity is also high.
The topography of the sample is often displayed in a format known as a grey scale, in which the image brightness at each pixel point is some function of the surface height at that point on the surface. For example, when the z feedback signal applied to the scanner causes it to cull the sample back (e.g., to compensate for the height of a peak on the surface) the corresponding data point on the display is painted bright. Conversely, when the sample is moved towards the probe (e.g., to compensate for the depth of a valley) the data point is painted dark. Each pixel on the display thus represents an x, y position on the sample and the z coordinate is represented by intensity. The z position can also be represented numerically or graphically with high precision.
As stated above, it is known to measure the deflection of the cantilever in an SFM by directing a laser beam against a smooth surface on the back of the cantilever and detecting changes in the position of the reflected laser beam as the cantilever is deflected. The shift in the laser beam position is normally detected by a bi-cell position-sensitive photodetector (PSPD). With conventional SFM""s, this detection circuitry generally obstructs an optical view of the probe positioned over the sample. application Ser. No. 07/668,886, filed Mar. 13, 1991, which is incorporated herein by reference, describes a deflection sensor in which the laser beam is reflected from a mirror positioned to a side of the cantilever so that the view from directly above the cantilever is not obstructed. That application also describes a system for kinematically mounting the mirror in the deflection sensor and a mechanism for kinematically mounting the head on the base.
These represent significant improvements over the prior art. However, a number of difficulties remain with prior art scanning probe microscopes, including the following:
1. In an SFM, the probe normally wears out and must be replaced after several samples have been scanned. Moreover, It is often desirable to change probes between samples to avoid contaminating the surface of a new sample with material accumulated on the tip from a previous sample surface. With the type of deflection sensor described above, the laser beam Rust be precisely directed to a very small area, on the order of 20 microns wide, on the back of the cantilever. Each time the cantilever is replaced, the laser beam must be readjusted so that it strikes the same position. Aligning the deflection sensor is a time-consuming procedure and typically requires a very precise position stage. For example, scanning a sample might take 30 minutes, and repositioning the laser spot might take an additional 15 minutes. Thus, a large portion of the time spent on a sample must be used to realign the deflection sensor after the probe has been replaced.
2. Different preamplification circuitry is required to amplify either the signal from the deflection sensor in an SFM or the tunneling currents from the tip in an STM. These preamps must be located in the head, close to the source of their respective signals, to reduce noise pickup. Likewise, SFMs and STMs typically require different probes, also located in the head. In the prior art arrangements, a head is dedicated either to an SFM or to an STM. Consequently, the head must be disengaged and replaced in order to switch between SFM and STM operating modes. This is a time consuming procedure. Moreover, each head is an expensive component.
3. A bi-cell PSPD is typically used in the deflection sensor of an SFM to detect changes in light position caused by cantilever deflection. The sensitivity of bi-cells to these changes depends nonlinearly on the initial light position. Sensitivity is greatest when the light strikes the center of the PSPD, thereby producing a zero initial signal. As the light position moves off-center (i.e., an initial signal offset is present), sensitivity drops. If the initial offset is too large, the bi-cell cannot function, since light strikes only one of the cells. This nonlinear position response is further adversely affected by intensity variations across the width of the light spot. To minimize these effects, frequent and time-consuming adjustments to zero the initial signal offset are necessary before running the microscope and each time a probe is changed.
4. The coarse x, y stage in an SPM is often a stacked structure which has at least three levels: a fixed base, a y stage, and an x stage. This configuration has a relatively large mechanical loop, i.e., thermal and mechanical displacements in these individual stages are cumulative and can affect the spacing between the probe and sample. These displacements are a significant source of noise in the data. A configuration with a large mechanical loop may also be unstable.
5. Piezoelectric scanners inherently exhibit nonlinear behavior which includes hysteresis (where the scanner position for a given control voltage is a function of past history of movement), creep (where the scanner position gradually drifts in response to an applied voltage), and nonlinear response (where the scanner position is a nonlinear function of applied voltage). In addition, bending of a piezoelectric tube scanner is inherently associated with its lateral movement and causes it to tilt. These nonlinear effects contribute undesirably to the data image and require some means for scan correction. U.S. Pat. No. 5,051,646 describes a method to correct for these nonlinearities by applying a nonlinear control voltage to the piezoelectric scanner. However, this method is xe2x80x9copen loopxe2x80x9d, i.e., it does not use feedback and has no means to determine and correct the actual scanner motion due to the applied nonlinear input signal. Application Ser. No. 07/766,656, filed Sep. 26, 1991, which is incorporated herein by reference, describes a method of correcting for nonlinearities in the x, y lateral motion of the scanner that is xe2x80x9cclosed loopxe2x80x9d, i.e., it does use feedback. However, the method does not take into account the bending of a piezoelectric tube scanner, which causes tilt.
6. In typical SPMs the problem of hysteresis requires that each line of data in the raster scan be collected in the same direction, since data collected in the reverse direction includes the effects of hysteresis. As a consequence, each line of the raster scan must be traversed twicexe2x80x94once to collect data and once to return along the same path (or vice versa). The length of time necessary to generate an image is thus significantly greater than what it might be without hysteresis effects. Moreover, hysteresis problems prevent the use of data collected in the forward scan of a line to adjust the scan parameters before generating an image from scanning the line in the reverse direction.
7. Another source of error in the data image arises due to the thickness of the sample. As a piezoelectric tube scanner bends to thereby produce a lateral motion of a sample (or probe) mounted on it, the sample (or probe) moves in an arc-shaped path. As the thickness (vertical dimension) of the sample increases, a given input signal to the piezoelectric tube scanner therefore produces a larger horizontal translation of the surface of the sample.
8. In order to position the sample relative to the probe, it is useful to have both a coaxial (on-axis) and oblique view of the same using an optical microscope. These views provide means to monitor fine positioning of the sample relative to the probe. The coaxial view assists in positioning the probe over the feature of the sample to be measured. The oblique view permits accurate adjustment of probe orientation (for instance, cantilever tilt) relative to the sample surface. Conventional SPMs provide both these features; however, they are provided in two separate, annually operated microscopes, which are unwieldy to use. Obtaining these dual views is thus inconvenient.
9. In prior art SPMs the piezoelectric tube scanner cannot We operated at a rate greater than its resonant frequency. Above its resonant frequency, the response of the scanner to an input voltage signal is greatly reduced and out of phase with the input signal.
10. Prior art SPMs do not permit the adjustment of scanning parameters such as scanning rate or probe path in response to topographical features encountered by the probe.
In the scanning probe microscope of this invention, a package, which contains a probe, is kinematically mounted onto a cartridge, which in turn is kinematically mounted in the head. In order to switch probes, the cartridge is removed from the head and a new package, containing a new probe, is mounted onto the cartridge, which is then remounted in the head. The cartridge and package are thus both easily removed and replaced. The kinematic mounting techniques used ensure that the probe is positioned in the head within an accuracy of approximately 20 microns. This arrangement permits SFM and STM probes, and other types of probes, to be easily interchanged. Time-consuming adjustment to position the deflection sensor in an SFM is not required after probe replacement, as it is in the prior art.
The SFM probe consists of a flexible cantilever which projects from one end of a microfabricated chip. The chip is attached to a plate to form the package. This package also contains precisely aligned kinematic mounting points for securing it to the cartridge. The chip may be attached to the plate using integrated circuit (c) mounting techniques, a gluing process, or other methods. Alternatively, a combined, Integrated plate and chip can be microfabricated, and precisely aligned kinematic mounting points can be formed on it using lithographic means (for instance) to allow the package to be kinematically mounted on the cartridge. A chip or a package containing a chip may also be kinematically mounted directly in the head, thereby omitting the cartridge, or a chip may be kinematically mounted directly on the cartridge.
The deflection sensor of this invention uses a light beam deflection sensor to detect the angular movement of the light beam that occurs when the cantilever deflects. A linear position-sensitive photodetector (PSPD), i.e., an analog PSPD that can provide continuous linear information about the position of a light spot on the detector""s active surface, is used to detect this movement instead of a bi-cell PSPD. A linear PSPD has a highly linear, continuous response to the position of the incident light beam and is much more tolerant of an initial offset in light position. Frequent adjustments of the PSPD to zero the initial offset are no longer necessary as they are in the prior art. Since occasional adjustments may be necessary to center the light on the PSPD to minimize noise, a position adjustment mechanism for the linear PSPD is provided.
The head of the scanning probe microscope contains circuitry capable of preamplifying both SFM and STM signals, thereby eliminating the need for two different heads which must be switched when shifting between scanning force and scanning tunneling microscopy.
A single, non-stacked coarse x, y stage holds both the sample and scanner. The coarse x, y stage is slidably clamped to the base and is loaded against it at three contact points. It is normally held stationary by friction between the clamping surfaces and the base. When the position of the coarse x, y stage is adjusted, the three contact points slide across a smooth surface on the base, which may preferably be glass microscope slides. Horizontal translation of the coarse x, y stage is accomplished by two adjustment members which are oriented perpendicularly to one another. In a preferred embodiment, each adjustment member is a screw which is threaded through a fixed nut and driven by a stepper motor. An end of one screw is ball-tipped and makes a single point of contact with an edge of the coarse x, y stage. An end of the other screw makes contact with a pushing plate, which in turn makes two points of contact with another edge of the x, y coarse stage. The pushing plate slides on a rail mounted on the underside of the base. The x, y stage and pushing plate are biased against their respective contact points by loading springs. The configuration of six contact points which define the position relative to the base of the coarse x, y stage (three clamping points, two pushing plate points, one screw end) constitute a stable kinematic mount. The x and y stepper motors slide along respective rails as the screws are advanced and withdrawn. The fixed nuts represent reference points which are positioned so as to minimize the mechanical loop involved in positioning the coarse x, y stage. There are alternative means of kinematically mounting a single non-stacked coarse x, y stage to the base so as to minimize the mechanical loop of the configuration which will become apparent in what follows.
The coarse z stage comprises three adjustment members which are arranged in a triangular configuration and regulate the separation between the head and the coarse x, y stage. In a preferred embodiment, each adjustment member comprises a screw which is oriented vertically and threaded through a fixed nut in the base of the microscope. Each screw is driven by a stepper motor which slides along a rail as the screw is advanced or withdrawn. Each screw is ball-tipped, and the head is mounted kinematically on the three screws. This configuration allows both the elevation and tilt of the probe with respect to the sample to be adjusted.
The scanner (also referred to as the fine x, y, z stage) comprises a piezoelectric tube whose base is fixed to the coarse x, y stage and whose opposite end is free to move in response to an applied voltage. A quad-cell PSPD is mounted axially at the upper end of the piezoelectric tube and faces a light source (e.g., a light emitting diode (LED)) mounted at the base of the tube. As the upper end of the piezoelectric tube moves horizontally, the position of the light striking the quad-cell PSPD shifts. The quad-cell PSPD thus senses the x, y movement of the free end of the piezoelectric tube and thereby of a sample mounted on it. In addition, two bi-cell PSPDs are mounted on the outer surface of the piezoelectric tube such that they face two light sources (for example LEDs). The outputs from these PSPDs are added together to provide a z position signal which is insensitive to sample tilt (which occurs due to bending of the piezoelectric tube as described above). The signals from the axially-mounted PSPD and the twin surface-mounted PSPDs are used in closed feedback loops to correct for the nonlinear behavior of the tube scanner.
Each of the stepper motors in the coarse z stage trips a limit switch when it reaches a maximum vertical position. The limit switches are positioned so that the head is oriented horizontally relative to the base when all three limit switches are tripped. With all three limit switches tripped, i.e., with the probe raised to its maximum height above the sample, the thickness of the sample can be measured by then causing the stepper motors to retract the screws until the probe makes contact with the sample and recording the distance traversed. Measurement of the sample height is used to correct for the horizontal scanning error which arises due to the finite sample thickness. As noted above, this error results from the bending of the piezoelectric tube scanner as its free end, holding the sample, is displaced laterally in response to the input voltage. The measurement is used to adjust the x, y sensitivity of the piezoelectric tube scanner, which is expressed as a unit of scanner displacement per unit of applied bias (e.g., xcexcm/volts). Stepper motors are not required; any sufficiently well-calibrated and reproducible motor will suffice.
The sample thickness needs to be compared to that of a calibrating sample of the system (or reference surface). The reference surface is used to generate a value of the tube""s lateral sensitivity (xcexcm/V). The thickness of the calibrating sample (or reference height) is stored as the distance (or steps of the stepper motor) the z approach screws travel from the limit switches to the calibration sample or reference surface. An arbitrary sample""s thickness is measured relative to this calibration. A change in the sample thickness affects the calibration values of the scanner through a simple formula as described below. In this manner the sample thickness is measured and the sensitivity of the scanner is updated.
A combined on-axis optical view and an oblique optical view of the sample positioned relative to the probe are provided. Either of these optical paths is selected by positioning a motorized shutter under computer control. The dual optical views obtained using a motorized shutter, mirrors, and lenses and the means of switching between them advantageously eliminates the need for two separate optical microscopes. Also, the microscope lenses positioned in the two optical paths (which can be either objective or achromat, for instance) are moved under motor control to raise or lower the focal plane and thus focus the image under computer control.
The system includes a scanning probe microscope (SPM) graphical user interface which has a simultaneous on-screen optical view and SPM view for user reference. These views are also used to locate and define regions graphically for the next scan. The image from either of the two optical paths is focused on a conventional CCD camera by a computer controlled motorized zoom lens. The motorized zoom motor encoder allows automatic control of optical image magnification and optical image size. Calibration of the motorized zoom lens assembly permits accurate correlation of features in optical and SPM images. This eliminates the need for eyepieces for the optical microscope by displaying images on a video screen. Additionally, since the optical system is parfocal, the image magnification can be varied either by switching objective lenses (mounted on a conventional turret) or adjusting the motorized zoom lens and the image will remain focused.
On-screen views (both optical and SPM) are coupled to sample movement relative to the probe. Computerized motors (x,y, and z) and/or the scanner automatically position the sample in a scan region chosen by graphical means. A desired scan region on the sample can be chosen by graphically highlighting a portion of an optical image or an SPM image. Automatic positioning of the SPM can then be used to successively narrow the scan width and zoom in on a feature of interest. Thus manual adjustments to position for the next scan are no longer needed as they aren""t the prior art. The system places scan marks in the optical image to indicate SPM scan location, thereby creating a scan record. Features in the optical image and the SPM image can be accurately correlated.
The SPM of this invention uses an optical control process to automatically and quickly position the probe to within a few microns of the sample surface, by presetting the focal plane of the objective lens a few microns below the probe tip and then bringing this focal plane into coincidence with the sample. The three z stage motors and the motor coupled to the optical lens assembly are lowered in unison, moving the probe tip and the focal plane of the objective lens quickly down towards the sample, until software determines that the image of the sample surface is in focus. The z stage then slows down for final approach. This shortens the time required to bring the probe into proximity or contact with the surface, or to within the range of the fine x, y, z stage (the scanner).
This system also uses an optical control process to determine the tilt of a sample secured to a sample mount by bringing three different points on the surface into focus successively and determining the slope of the sample surface from this data. The sample slope is used for automatically adjusting the tilt of the head (and thereby the probe) so as to make it parallel to the sample surface. This tilt information can also be used to adjust scanning parameters or image display parameters that will remove this overall slope from images of the surface. Thus, this process can determine sample slope and probe tilt. (This slope can be due, for instance, to a crooked sample mount.)
Data image buffers in the user interface of the system are used to automatically transfer data between data acquisition and image processing modes, thus conserving permanent storage space. Buffers are displayed on-screen for visual reference and can be brought into an active window for image processing. The buffers can include data collected in real-time or data brought in from a database. Porting the buffered images automatically between the data acquisition and image processing modes gives the user much greater flexibility to analyze data in real-time, to quickly extract quantitative information, and to do image processing to determine if it will be worth saving permanently.
The user interface of this system provides a fast one-dimensional FFT (Fast Fourier Transform) performed on a live line trace of the data (i.e., a digital oscilloscope). Providing a live one-dimensional FFT allows the user to extract quantitative information without importing the image to an analysis program. Furthermore, the ability of the controller to perform FFT or other analyses on line data in real time allows the controller to use the results of the analysis to optimize the present scanning and feedback parameters of the SPM system. More generally, the system analytically detects undesirable outputs such as a mechanical resonance in the scanning data and then changes the scanning parameters (such as speed) so as to avoid exiting the resonance.
In another application of the ability to perform a one-dimensional FFT in real time, the user can display the live line trace and its one-dimensional FFT, and can also display the logarithm of the error signal. The latter is a useful capability in STM, where the signal (tunneling current) depends exponentially on the spacing between the tip and the sample. The user interface can perform the one-dimensional FFT on an arbitrary line of the data image, including an image retrieved from a database, using high and low pass filters which are graphically applied to the line using standard graphical user interface features such as cursors. The resultant filtered line is displayed in real time.
The interface of this system also provides a two-dimensional FFT and applies high and low pass filters to a reduced region of the sample for increased processing speed before applying the FFT to the entire data image. The use of, for instance, cursors to adjust filtering parameters and the display in real time of the calculation result makes using the variable and pass filter very intuitive and easy.
The interface of this system also facilitates optimization of parameters for 3-dimensional rendering of the data image. This rendering is the manner of displaying 3-dimensional data in the form that gives the illusion of depth, slope, shading, etc. on a computer screen. It uses a graphic to show the effect in real time of varying parameters for 3-dimensional rendering. Optimized parameters are then applied to the data image. This significantly shortens the iteration process required to achieve optimal 3-dimensional rendering. Any graphic can be used for this purpose, such as an artificial structure having simple geometries or a reduced data set such as data from some fraction of the image data to be processed.